The invention concerns a method for determination of an irreversible stretch and of a dynamic modulus of elasticity from a paper web. The invention also concerns a method for the control of a papermaking process.
It is known from the prior art that, in manufacture of paper and board material, it is necessary to monitor the properties of the web material during the process of manufacture. For the quality control, a number of prior-art on-line or off-line methods are available. As a result of a determination of the properties of the web material, it is possible to regulate different parameters that are used in papermaking. The aim of the regulation is a final product of optimal quality and good runnability of the machines.
Earlier, the significance of the cross-direction tension profile of a paper web both for the quality of the ultimate product and for the runnability of the machines has been realized. In the applicant""s FI Patent No. 94,066, there is a thorough discussion of the significance of the tension profile for the smooth running of the process of manufacture and for the quality of the ultimate product.
In the applicant""s FI Patent No. 80,522, a method and an equipment for measurement of the tension of a web are described. In said solution, the tension of the web is measured by, in the vicinity of the moving web, fitting a measurement rib, which has a face that is curved in the running direction of the web and in which rib there are pressure metering detectors placed in holes. An air cushion is formed between the moving web and the curved face, in which cushion the air pressure is directly proportional to the tension of the web. The tension of the web is measured indirectly by metering the pressure in the air cushion.
In the FI Patent No. 62,419, a method for measurement of a tension profile is described, which method is based on the speed of propagation of a plane wave. In this method, an acoustic impulse is produced in the web, and the travel time of the impulse is measured by means of a microphone.
In the U.S. Pat. Nos. 2,755,032 and 3,057,574, mechanical and pneumatic equipments of measurement and regulation have been suggested for measurement of the tension profile of a web. In these apparatuses, the web is pressed by means of two shoes, which produce a change in the running direction of the web, and, at the same time, a force directly proportional to the tension of the web is applied to the shoes.
Even though a number of on-line methods are available for examination of the tension profile and of other properties of a web material, it has been difficult to determine the irreversible stretch and the dynamic modulus of elasticity during the process. As a rule, these quantities are determined under laboratory conditions. For this purpose, separate samples must be taken from the web material, and the result of determination of the quantities is obtained only after a delay. Thus, it takes a great deal of time before the necessary adjustments in the process can be made on the basis of the results obtained from the laboratory.
The problems stated above and involved in the prior art have constituted a reason for the present invention.
The object of the present invention is to provide a novel method for determination of an irreversible stretch and of a dynamic modulus of elasticity from a paper web. The aim of the method in accordance with the method is to provide a possibility for rapid adjustment of the regulation parameters employed in the manufacture of paper after determination of the quantities concerned.
In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention for determination of an irreversible stretch and of a dynamic modulus of elasticity from a paper web is mainly characterized in that the method comprises the following steps:
the paper web is subjected to an on-line measurement of its cross-direction tension profile at two or more different tension levels,
a curve is fitted to the number of points of tension measurement carried out at each location of measurement in the cross direction of the paper web,
the irreversible stretch is determined from each fitted curve, and
the dynamic modulus of elasticity is determined from each fitted curve.
The method in accordance with the invention for the control of a papermaking process is mainly characterized in that, in said method, the method in accordance with the invention for determination of an irreversible stretch and of a dynamic modulus of elasticity from a paper web is employed for regulation of the dynamic modulus of elasticity and/or of the irreversible stretch.
In carrying the method in accordance with the invention into effect, it is possible to use apparatuses of measurement of the cross-direction tension profile of a web which are known from the prior art and which have possibly already been installed in the paper machine. The tension profile is determined across the entire width of the paper web at two or more different tension levels, and the stretch between targets is determined as a measurement of difference in speed.
At each of the different points of measurement of the tension profile, the measurement results of a number of measured tension profiles are fitted together. From the fitted result thus formed, it is possible to determine the irreversible stretch and the modulus of elasticity of the paper web.
A particular advantage of the present invention is the rapidity of the method of measurement as well as the possibility of integrating the measurement procedure on-line in the rest of the control system of the paper machine.